1. Field of the Invention
The present invention relates to a fixing device for an electrographic (colour) printing apparatus in which for example two images on both sides of a receiving member can be fixed in one step.
2. Description of the Prior Art
In an electrophotographic black-and-white printing machine, a photoconductive member is charged to a substantially uniform potential to sensitize the surface thereof. The charged portion of the photoconductive member is image-wise exposed. Exposure by irradiation of the charged photoconductive member selectively dissipates the charge thereon in the irradiated areas. As a result an electrostatic latent image is recorded on the photoconductive member corresponding to the informational areas contained in the original document being reproduced. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing a charged toner brush into contact therewith. A developed toner image is formed on the photoconductive member. The toner image may be subsequently transferred in one or more steps to a receiving member. The receiving member carrying the toner image is then heated to permanently affix the toner image thereto in image configuration. A toner image can also be obtained using other electrographic systems. In direct electrostatic printing for example switchable aperture electrodes image-wise regulate the toner transfer through the printhead to the receiving medium. Multicolour electrophotographic printing is substantially identical to black and white printing described hereinbefore. However, rather than forming a single latent image on the photoconductive surface, successive latent part-images corresponding to different colours are recorded thereon. Starting from colour separation information obtained by scanning or computing an image according to the additive colour system using red, green and blue (RGB), the part-image forming data is calculated for obtaining an electrostatic latent image which is preferable developed with a toner of the subtractive colour system using magenta, yellow and cyan toner.
This process is repeated at the image forming station or duplicated at plural image forming stations for the respective subtractive colour system toners (YMC). A fourth image using black toner can be included to provide better image reproduction. Each single colour component toner image is transferred to the receiving member sheet in superimposed registration with the prior toner image, thereby creating a multilayered toner image on the receiving member. Thereafter, the multilayered toner image is permanently fixed to the receiving member creating a colour copy or print.
The fixing is usually done by a fusing apparatus comprising two pressure rollers exerting pressure to the receiving member which is fed in between the rollers. Due to the pressure the toner compacts and adheres to the receiving member. Usually at least one of the rollers is heated in order to cause melting of the toner providing fast and high quality fixing of the copy. This provides a capability of a high throughput fixing apparatus. A problem herein is that the melted toner not only adheres to the receiving member, but also tends to adhere to the fixing rollers which exert pressure and heat to the receiving member. This causes smearing of the rollers, resulting in image quality defects in copies made hereafter.
To avoid these problems and to exert a uniform pressure, the fixing rollers are usually made of a rubber material having a low affinity for the toner composition. To even further diminish the toner offset to the rollers a releasing agent is applied to the fixing rollers. This is usually a type of silicone fuser oil forming a thin layer on the fixing rollers. This oil usually is applied to the fuser roller by a contacting oil application roller on which a constant oil film is applied by an oil applicator and metering blade. The oil ensures no tacking of the toner to the fixing roller. Such a system is described in U.S. Pat. No.5,504,566 wherein an oil application system uses a dual metering blade. However a small portion of the oil applied to the fixing roller is also transferred to the final copy, which is fixed in the fuser. This thin oil film results in a light gloss visible in the final copy.
This oil offset also results in a change in the amount of oil present on the fixing rollers even if a fresh supply is provided. In order to maintain good toner offset properties, the amount of oil on the fusing rollers has to be maintained at a certain level.
Another problem arises when the thickness of the oil film deposited on the copy varies within one page. The visible variation of gloss results in a degraded image quality to the viewer. In order to counteract these problems a rapid variation of the oil film thickness has to be avoided. It can be seen that there is still a need for a better control of silicone oil application to maintain good offset properties and a high image quality.
When the fuser is activated without actually fusing copies, the oil application system ensures a maximum of oil deposited upon the fuser rollers.
If the fuser is used to fuse a continuous web of paper, the oil film on the rollers diminishes each revolution until a minimum value is reached. At this point the amount of oil offset on the paper equals the amount of oil supplied by the applicator system. This maximum and minimum value can be calculated and measured. Normal copy or print operation however cannot be compared to either situation. In between copies the amount of oil on the rollers increases whereas it decreases when a sheet is fed through the nip. This decrease is not continuous as the length of the paper sheet normally exceeds the circumference of the fuser roller. During the first revolution of the fuser roller in contact with the leading portion of the receiving sheet, the fuser roller is fully oiled and a certain amount of oil transferred to the paper. During the second revolution of the fuser roller in contact with the second portion of the receiving paper there is less oil on the roller and a smaller amount of oil is transferred to the paper. During further revolutions the amount of oil on the roller will diminish to the minimum value. The transition of two areas having a different oil coverage occurs in discrete steps clearly visible to the viewer.
A further problem which arises is the fact that several systems using metering blades or wicks cause premature wear of the fusing rollers due to sliding contacts.
An oil application system in a fuser is normally provided only for the roller contacting the toner image. Till now no oil application system is provided for fusing a duplex copy wherein the two toner images on both sides of the receiving layer are fused simultaneously.